Manufacturing method for QPM wavelength converter elements, QPM wavelength converter element, and medical laser apparatus using it

ABSTRACT

A manufacturing method for quasi phase matching (QPM) wavelength converter elements using crystal quartz as a base material in which twins are periodically induced, comprises a step of periodically inducing the twins by applying a stress onto a crystal quartz substrate as the base material so that an angle e of a direction in which the stress is applied relative to a Z axis of the crystal quartz is 60°&lt;θ&lt;90°.

FIELD OF THE INVENTION

[0001] The present invention relates to a manufacturing method for quasiphase matching (QPM) wavelength converter elements (devices) usingcrystal quartz as the base material (host material), a QPM wavelengthconverter element (device), and a medical laser apparatus using it.

DESCRIPTION OF RELATED ART

[0002] In recent years, many research and development attempts on solidlasers using wavelength converter elements have been made. Morerecently, in particular, the establishment of a high voltage applicationmethod has much facilitated fabrication of QPM wavelength converterelements using ferroelectric crystals. This has enabled high efficiencywavelength conversion to be accomplished in the visible to infraredwavelength ranges.

[0003] For wavelength conversion in the ultraviolet wavelength range,fabrication of QPM wavelength converter elements from BaMgF₄ crystals,which are ferroelectric, is attempted. However, the BaMgF₄ crystals havea very low effective nonlinear constant. For this reason, fabrication ofQPM wavelength converter elements from crystal quart (SiO₂), whoseeffective nonlinear constant is about 10 times as great as that of theBaMgF₄ crystals, is under study.

[0004] It has to be noted here that in the crystal quartz, as it is anon-ferroelectric, the application of high voltage does not work forfabricating a QPM structure. Therefore, an alternative method ofinducing periodic twins (hemitropes) by applying a stress to the crystalquartz and thereby realizing a polarity-inverted structure has beenproposed. This results in a change in the sign of nonlinear opticalconstant dll among the twins, which enables QPM in the period of twinalignment (arrangement).

[0005] Incidentally, the application of the stress to the crystal quartzwas previously considered to cause the twins to grow along the Z a ofthe crystal quartz. For this reason, the stress application was socarried out that the angle θ of the stress application relative to the Zaxis of the crystal quartz is 0°<θ<60°. Further, in inducing the twinsin the crystal quartz, the stress application was performed whileuniformizing the temperature distribution of the crystal quartz in thevicinity of the phase transition temperature (573° C.) of the crystalquartz.

[0006] In this way, fabrication of QPM wavelength converter elementswhich would function as wavelength converter elements when the directionof the Z aria of the crystal quartz is made substantially orthogonal tothe incident light vector is attempted by forming the polarity invertedstructure by inducing the periodic twins.

[0007] However, the conventional manufacturing method involves a problemthat the aspect ratio in the growth of the twins is extremely low inaddition to low controllability of the twins. As a result, it has beenimpossible to obtain practical usable QPM wavelength converter elementsin bulk using the crystal quartz as the base material.

SUMMARY OF THE INVENTION

[0008] The present invention has been made in view of the abovecircumstances and has an object to overcome the above problems and toimprove the control of the twins in the crystal quartz, realize thegrowth of the twins in a high aspect ratio, and provide a manufacturingmethod for QPM wavelength converter elements permitting in particularfor practical use wavelength conversion to the ultraviolet range, such aQPM wavelength converter element, and a medical laser apparatus usingit.

[0009] Additional objects and advantages of the invention will be setforth in part in the description which follows and in part will beobvious from the description, or may be learned by practice of theinvention The objects and advantages of the invention may be realizedand attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

[0010] To achieve the purpose of the invention, there is provided amanufacturing method for quasi phase matching (QPM) wavelength converterelements using crystal quartz as a base material in which twins areperiodically induced, comprising: a step of periodical inducing thetwins by applying a stress onto a crystal quartz substrate as the basematerial so that an angle e of a direction in which the stress isapplied relative to a Z axis of the crystal quartz is 60°<θ<90°.

[0011] According to another aspect, the present invention provides amanufacturing method for quasi phase matching (QPM) wavelength converterelements using crystal quartz as a base material in which twins areperiodically induced, comprising: a stress application step ofperiodically inducing the twins by applying a stress onto a crystalquartz substrate as the base material; and a beat treatment step ofkeeping a temperature between two planes of the crystal quartz substrateorthogonal to a direction in which the stress is applied at or below aphase transition temperature of the crystal quartz and creating atemperature difference between the two planes.

[0012] According to another aspect, the present invention provides aquasi phase matching (QPM) wavelength converter element whose basematerial is crystal quartz in which twins are periodically induced byapplying a stress, wherein interfaces of the twins are formed in a planecontaining a Y axis of the crystal quartz and the twins are formed in adirection of a Z axis of the crystal quartz periodically

[0013] According to another aspect, the present invention provides amedical laser apparatus comprising: a laser light source; and awavelength converter element for converting the wavelength of a laserbeam from the laser light source, wherein the wavelength converterelement is the QPM wavelength converter element mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated in andconstitute a part of this specification illustrate an embodiment of theinvention and, together with the description, serve to explain theobjects, advantages and principles of the invention.

[0015] In the drawings,

[0016]FIG. 1 illustrates a schematic structure of a quasi phase matchingwavelength converter element (QPM crystal quarts) in an embodiment ofthe invention and wavelength conversion using the QPM crystal quartz;

[0017]FIG. 2 illustrates a schematic structure of QPM crystal quartz inanother embodiment and wavelength conversion using the QPM crystalquarts

[0018]FIGS. 3A to 3C are views to explain a manufacturing method for theQPM crystal quartz in the embodiment;

[0019]FIG. 4 shows a result of computation of angle dependence ofcoercive stress;

[0020]FIG. 5 is a view to explain a manufacturing method for the QPMcrystal quartz in another embodiment; and

[0021]FIG. 6 illustrates a schematic structure of a medical laserapparatus using the QPM crystal quartz in the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] A preferred embodiment of the present invention will be describedbelow with reference to the accompanying drawings. FIG. 1 illustrates aschematic structure of a quasi phase matching wavelength converterelement (QPM crystal quartz), which is the preferred embodiment of theinvention and wavelength conversion using the QPM crystal quartz.

[0023] A quasi phase matching wavelength converter element hereinafterreferred to as QPM crystal quartz) 10 uses crystal quartz 11 as the basematerial Crystal quartz has many useful features including excellentchemical stability, a high damage threshold, transparency up to theultraviolet region of 150 nm and a lower cost than other crystals. Thus,the crystal quartz has advantages as a material for wavelength converterelements for the generation of ultraviolet rays.

[0024] Twins in the crystal quartz were previously considered to grow inthe direction of the Z axis of the crystal quartz when the twine in thecrystal quartz were caused to grow by applying a stress. However,experimental findings by the present inventors have revealed that twinsat first grow in the direction of the Y axis of the crystal quartz,followed by growth in the direction of the Z axis.

[0025] This indicates that the QPM crystal quartz 10 has the followingstructure. In the QPM crystal quartz 10, periodic twins 12 are inducedin the direction of the Z axis of the crystal quartz 11 as the basematerial, resulting in the formation of a structure whose polarity isperiodically inverted. The interfaces 12 a of the twins 12 are formed ina plane containing the Y axis of the crystal quartz 11. By bringing afundamental wave beam 20 into incidence in the direction of the Z axisof this QPM crystal quartz 10, a wavelength-converted beam 21, which isthe second harmonic of the fundamental wave beam 20, is caused to beemitted.

[0026] The incidence vector of the fundamental wave beam 20 shouldpreferably be, but not absolutely required to be, parallel to thedirection of the Z axis of the crystal quartz 11 (substantiallyorthogonal to the plane of the ZY axes in FIG. 1). Since QPM is achievedin the crystal quartz 11 because the sign of its nonlinear opticalconstant dll is periodically inverted, the wavelength-converted beam 21can be taken out if the polarized beam 20 a of the fundamental wave beam20 at least has an X axis component. For practical purposes, where theangle formed by the direction of the Z axis and the incidence vector ofthe fundamental wave beam 20 is represented by a, a should preferably be0°≦α≦30°.

[0027] Although the interfaces 12 a are supposed to contain the X axisof the crystal quartz 11 too in their planes as illustrated in FIG. 1,the interfaces 12 a may as well be so formed as not to contain the Xaxis of the crystal quartz 11 in their planes as illustrated in FIG. 2.In taking out the wavelength-converted beam 21, however, the polarizedbeam 20 a of the fundamental wave beam 20 needs to have the X axiscomponent. Thus for this reason, so that the periodic twine 12 be formedin the direction of the Z axis, the angle v formed by the interfaces 12a and the X axis is at least prevented from being perpendicular.

[0028] Next will be dewed the manufacturing method of the QPM crystalquartz 10 with reference to FIG. 3. First as shown in FIG. 3A, the angleθ of the direction in which the stress is applied relative to the Z axisof the crystal quartz is set to 60°<θ<90°, the cut-out orientation Crelative to the Y axis is of the crystal quartz is set to 0°<θ<30°. Thereason why the angle θ of the direction in which the stress is appliedrelative to the Z axis of the crystal quartz is set to 60°<θ<90° is thattwins have been found to grow in the direction of the Y axis of thecrystal quartz. Preferably the angle e of the direction in which thestress is applied relative to the Z axis of the crystal quartz should be80°≦θ≦88° (2°<θ′<20°). Incidentally in this embodiment, a crystal quartzsubstrate 30 of 3 mm in thickness cut in an orientation of 56 from the Yaxis (=850 from the Z axis) is used.

[0029] Now, FIG. 4 shows the result of computation of theangle-dependence of coercive stress at a temperature of 400° C. In thediagram, the curve in one-dot chain line represents the stress level atwhich the growth of twins begins and that in solid line is where thegrowth of twins is completed. Previously, the angle θ of the directionof stress application relative to the direction of the Z axis was set to0°<θ≦60°, e.g. θ=13°. This was because twins were considered to grow inthe direction of the Z axis. By contrast to it, in this embodiment ofthe invention the angle θ of the direction of stress applicationrelative to the direction of the Z axis is set to 60°<θ<90°. This isbecause twins have been found to grow in the direction of the Y axis ofthe crystal quartz.

[0030] Next, machining is done to form on the surface of the crystalquartz substrate 80 a stepped structure 31 having level gaps in a periodof realizing the desired wavelength conversion. The stepped structure 31can be formed by photolithography. The depth of the step is, e.g., 2 μm.This stepped structure 31 may be on the first heater block 40 side onwhich the stress would be applied to the crystal quartz substrate 80.

[0031] Next, the crystal quartz substrate 80, over which the steppedstructure 31 is formed, is sandwiched between the first heater block 40and a second heater block 41 as shown in FIG. 3B, and a uniform uniaxialvertical stress is applied with a stress applying device 43. In thisprocess, the temperature T1 of the first heater block 40 arranged on thestepped structure 81 side and the temperature T2 of the other secondheater block 41 are kept at or below the phase transition temperature(5730C) Also, in order to keep the temperature T1 higher than thetemperature T2 (T1>T2), a temperature difference ΔT is created betweentwo planes orthogonal to the direction of stress application. Forinstance, the temperature difference ΔT of 176° C. is created by settingT1 to 375° C. and T2 to 200° C. The heater blocks 40 and 41 arecontrolled by a control device 42 to be individually variable intemperature.

[0032] Then, in a state in which the temperature difference ΔT iscreated at a level below the phase transition temperature between thetwo planes orthogonal to the direction of stress application, theuniform vertical stress is applied with the stress applying device 43.This causes anisotropic twins (twins growing in the direction of aspecific axis of the crystal quartz) reflecting the level gap of thestepped structure 31 over the crystal quartz substrate 30 in thedirection of the Y axis as shown in FIG. 3C, and the crystal quartzsubstrate 30 having periodic twins in the direction of the Z axis isobtained. By cutting and grinding this crystal quartz substrate 30 sothat the end face of the crystal quartz and the Z axis are substantiallyorthogonal to each other, the QPM crystal quartz 10 illustrated in FIG.1 is obtained.

[0033] In an experiment by the present inventors, when a stress wasapplied while the temperature was simply raised to the vicinity of thephase transition temperature without differentiating the temperaturebetween the two planes orthogonal to the direction of stressapplication, isotropic twins (twins growing at random withoutdistinction between the Y axis and the Z axis of the crystal quartz)emerged, and sometimes no twins reflecting the level gap were inducedfrom the stepped structure 31 side. By contrast, when the temperaturedifference ΔT was created between the two planes orthogonal to thedirection of stress application, twins reflecting the level gap wereinduced from the stepped structure 31 side where temperature was higher,and they grew long toward the lower temperature side.

[0034] Further in the experiment by the present inventors, when thetemperature T2 on the second heater block 41 side was gradually raisedwith the temperature T1 on the first heater block 40 side kept at 375°C., anisotropic twins grew until n reached 200° C. Then, the anisotropictwins grew longer with a rise in the temperature T2. At T2=226° C.,however, isotropic twins began to grow from the lower temperature side(the second heater block 41 side). Therefore, no twins are induced fromthe other side of the stepped structure 31, and in order to selectivelycause anisotropic twins to grow from the stepped structure 31 side, thetemperature T2 on the lower temperature side (the second heater block 41side) should be T2<225° C. Preferably, it should be T2≦220° C.

[0035] On the other hand, when the temperature on the higher temperatureside (the first heater block 40 side) was gradually raised from 375° C.with T2 being kept at 200° C., only anisotropic twins grew until T1reached. 450° C. However, at TX 475° C., isotropic twins instead ofanisotropic twins came to grow predominantly. Therefore, in order toselectively cause anisotropic twins to grow from the stepped structure31 side, the temperature T1 on the higher temperature side (the firstheater block 40 side) should be higher than T2 and 250° C.<T1<475° C.Preferably, T1 should be 300° C.≦5 T1≦470° C. More preferably, T1 shouldbe 375° C.≦T1≦450° C.

[0036] Although the diagram of FIG. 5C illustrates a case in which thetwins 12 growing in the direction of the Y axis penetrate the crystalquartz substrate 30 as far as its under side (the other side of thestepped structure 31), they may as well be caused to grow to some middlepoint as shown in FIG. 5 instead of letting the twins 12 penetrate thecrystal quartz substrate 30 as far as its under side (the other side ofthe stepped structure 31) Since the twins grow in the direction of the Zaxis after they have grown in the direction of the Y axis, they will beeasier to control if they are now allowed to penetrate the crystalquartz substrate 30 as far as its under side. The length of the twinecan be controlled according to the conditions of the temperatures T1 andT2. If the twins are to be relatively short, T2 should be lower than200° C. and ΔT, larger than 175° C. Advisably, in using the QPM crystalquartz 10 obtained by not letting the twins penetrate the crystal quartzsubstrate 30 as far as its under side, the fundamental wave beam shouldbe let pass the region in which the twins are formed.

[0037] A description of a laser apparatus 150 using the QPM crystalquartz 10 obtained as described above will follow, with reference toFIG. 6. FIG. 6 illustrates a schematic structure of the laser apparatus150. Here it will be described, by way of example, with reference to amedical laser apparatus for cornea ablation using a wavelength-convertedbeam in the ultraviolet region.

[0038] An Nd:YAG solid laser light source 101, wavelength converterelements 102, 103 and 104, and a pair of prisms 105 a and 105 b arearranged in a laser light source unit 100 provided in the laserapparatus 150. The solid laser light source 101 emits a pulse laser beamof 1064 nm The wavelength converter element 102 generates a convertedbeam of 532 nm in wavelength by converting a fundamental wave beam of1064 nm in wavelength into its second harmonic. The wavelength converterelement 108 generates a converted beam of 266 nm in wavelength byconverting the converted beam of 532 nm to its second harmonic. Thewavelength converter element 104 generates a converted beam of 213 nm inwavelength, which is the sum-frequency beam of the wavelength 1064 nm ofthe components not converted by the wavelength converter element 102 andthe wavelength 266 nm converted by the wavelength converter element 103.Here, the QPM crystal quartz 10 shown in FIG. 1 are used as thewavelength converter elements 108 and 104 for wavelength conversion intothe ultraviolet region. As the wavelength converter element 102, a KTPcrystal or the like can be used, but the same QPM crystal quartz 10 canbe used as well.

[0039] The prism 105 a separates the laser beams of different wavelengthfrom one another. Out of the laser beams separated by the prism 105 a,that of 213 nm in wavelength comes incident on the prism 106 b, andother beams are shielded by a shield element (not shown). The laser beamof 213 nm in wavelength, as the beam for therapeutic use, is adjusted inthe output direction, and emitted from the laser light source unit 100.

[0040] A guiding optical system 110 is provided with a scanning opticalsystem consisting of two galvano mirrors 111 and 112 and a dichroicmirror 113. The dichroic mirror 113 has a characteristic of reflecting alaser beam of 213 nm and transmitting visible beams. A laser beamscanned at high speed by the two galvano-mirrors 111 and 112 is thenreflected by the dichroic mirror 113 to be guided to a cornea Ec of thepatient's eye. Although the optical system on the optical path from thelaser light source unit 100 to the galvano-mirror 111 is notillustrated, a minor for reflection the laser beam, an optical systemfor shaping the laser beam into a circular spot, and a correctiveoptical system for correcting its energy distribution are appropriatelyarranged. Advisably, the spot size of the laser beam in this laserapparatus 150 should be about 1 mm on the cornea Bc. Over the dichroicmirror 113 is arranged an observation optical system 120.

[0041] A corneal surgery using thins laser apparatus 150 will be brieflydescribed below. When data on the corneal surgery is entered into thelaser apparatus 150, a control unit (not shown) obtains control data forlaser irradiation on the basis of the corneal surgery data. Forinstance, where myopia is to be corrected, a combination ofsuperposition of a pulse laser and the number of pulses (duration ofirradiation) is used as the control data for laser irradiation forablation which is to be deep in the central part of the cornea Ec andprogressively shallower toward the periphery. An ultraviolet beam of 213nm is supplied from the laser light source unit 100 by the wavelengthconversion described above, and scanning operations by thegalvano-mirrors 111 and 112 controlled on the basis of the control dataguide the laser beam onto the cornea Ec. This causes the cornea Ec to beablated into the desired shape.

[0042] As described above, wavelength conversion by the QPM crystalquartz 10 is applicable in particular to laser apparatuses which conductwavelength conversion to laser beams in the ultraviolet region, andsuitable for use in laser apparatuses for medical use.

[0043] As described above, according to the present invention, controlof twins in crystal quartz can be improved and the growth of twinsrealized in a high aspect ratio. Furthermore, it enables wavelengthconversion of ultraviolet rays for practical purposes and its suitableapplication to laser apparatuses for medical use.

[0044] While the presently preferred embodiment of the present inventionhas been shown and described, it is to be understood that thisdisclosure is for the purpose of illustration and that various changesand modifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A manufacturing method for quasi phase matching(QPM) wavelength converter elements using crystal quartz as a basematerial in which twins are periodically induced, comprising: a step ofperiodically inducing the twins by applying a stress onto a crystalquartz substrate as the base material so that an angle G of a directionin which the stress is applied relative to a Z axis of the crystalquartz is 60°≦θ≦90°.
 2. The manufacturing method according to claim 1,wherein the angle θ is 80°≦θ≦88°.
 3. The manufacturing method accordingto claim 1, wherein the crystal quartz substrate is cut out so that acutout orientation θ′ relative to a Y axis of the crystal quartz is0°≦θ′≦30°.
 4. The manufacturing method according to claim 2, wherein thecrystal quartz substrate is cut out so that a cut-out orientation θ′relative to a Y axis of the crystal quartz is 2°≦θ′≦20°.
 5. Themanufacturing method according to claim 1, further comprising a step offorming, on a surface on the crystal quartz substrate, a steppedstructure with a periodicity for realizing a desired wavelengthconversion.
 6. The manufacturing method according to claim 5, wherein atthe forming step, the stepped structure is formed on a stressapplication side of the surface on the crystal quartz substrate.
 7. Amanufacturing method for quasi phase matching (QPM) wavelength converterelements using crystal quartz as a base material in which twins areperiodically induced, comprising: a stress application step ofperiodically inducing the twins by applying a stress onto a crystalquartz substrate as the base material; and a heat treatment step ofkeeping a temperature between two planes of the crystal quartz substrateorthogonal to a direction in which the stress is applied at or below aphase transition temperature of the crystal quartz and creating atemperature difference between the two planes.
 8. The manufacturingmethod according to claim 7, further comprising a step of forming, on asurface on the crystal quartz substrate, a stepped structure with aperiodicity for realizing a desired wavelength conversion.
 9. Themanufacturing method according to claim 8, wherein at the forming step,the stepped structure is formed on a stress application side of thesurface on the crystal quartz substrate.
 10. The manufacturing methodaccording to claim 8, wherein T1>T2 holds at the heat treatment stepwhere T1 is a temperature of a stepped structure side of the two planesand T2 is a temperature of the other side.
 11. The manufacturing methodaccording to claim 10, wherein at the heat treatment step, thetemperature T1 is kept higher than 250° C. and lower than 475° C. andthe temperature T2 is kept below 225° C.
 12. The manufacturing methodaccording to claim 10, wherein at the heat treatment step, thetemperature T2 is so controlled as to stop the twins, which grow fromthe stepped structure side, from growing in a direction of a Y axis ofthe crystal quartz before they reach the other side.
 13. Themanufacturing method according to claim 12, wherein at the heattreatment step, the temperature T2 is kept below 200° C. and thedifference between the temperatures T1 and T2 is kept greater than 175°C.
 14. A quasi phase matching (QPM) wavelength converter element whosebase material is crystal quartz in which twine are periodically inducedby applying a stress, wherein interfaces of the twins are formed in aplane containing a Y axis of the crystal quartz and the twins are formedin a direction of a Z axis of the crystal quartz periodically.
 15. TheQPM wavelength converter element according to claim 14, wherein theinterfaces of the twins are formed in a plane containing also an X axisof the crystal quartz.
 16. The QPM wavelength converter elementaccording to claim 14, wherein an angle α formed by the direction of theZ axis of the crystal quartz and an incidence vector of a fundamentalwave beam is 0°≦α≦80°.
 17. A medical laser apparatus comprising: a laserlight source; and a wavelength converter element for converting thewavelength of a laser beam from the laser light source, wherein thewavelength converter element is the QPM wavelength converter elementaccording to claim 14.